fast-import: add '--signed-tags=<mode>' option

fast-import: add '--signed-tags=<mode>' option

Recently, eaaddf5791 (fast-import: add '--signed-commits=<mode>'
option, 2025-09-17) added support for controlling how signed commits
are handled by `git fast-import`, but there is no option yet to
decide about signed tags.

To remediate that, let's add a '--signed-tags=<mode>' option to
`git fast-import` too.

With this, both `git fast-export` and `git fast-import` have both
a '--signed-tags=<mode>' and a '--signed-commits=<mode>' supporting
the same <mode>s.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>